U.S. patent application number 14/415124 was filed with the patent office on 2015-07-23 for polarization diverse wavelength selective switch.
This patent application is currently assigned to Finisar Corporation. The applicant listed for this patent is Finisar Corporation. Invention is credited to Steven James Frisken.
Application Number | 20150208143 14/415124 |
Document ID | / |
Family ID | 49949247 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150208143 |
Kind Code |
A1 |
Frisken; Steven James |
July 23, 2015 |
Polarization Diverse Wavelength Selective Switch
Abstract
Described herein is a wavelength selective switch (WSS) type
optical switching device (1) configured for switching input optical
beams from input optical fiber ports (3, 5 and 7) to an output
optical fiber port (9). Device (1) includes a wavelength dispersive
grism element (13) for spatially dispersing the individual
wavelength channels from an input optical beam in the direction of
a second axis (y-axis). The optical beams propagate from input
ports (3, 5 and 7) in a forward direction and are reflected from a
liquid crystal on silicon (LCOS) device (11) in a return direction
to output port (9). The input optical beams are transmitted through
a port selecting module (21), which provides polarization diversity
to device (1) and provides capability to restrict optical beams
returning from LCOS device (11) from being coupled back into input
ports (3, 5 and 7).
Inventors: |
Frisken; Steven James;
(Vaucluse, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Finisar Corporation |
Horsham |
PA |
US |
|
|
Assignee: |
Finisar Corporation
Horsham
PA
|
Family ID: |
49949247 |
Appl. No.: |
14/415124 |
Filed: |
July 18, 2013 |
PCT Filed: |
July 18, 2013 |
PCT NO: |
PCT/US13/51064 |
371 Date: |
January 15, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61673580 |
Jul 19, 2012 |
|
|
|
Current U.S.
Class: |
398/48 |
Current CPC
Class: |
H04Q 2011/0039 20130101;
G02B 6/356 20130101; H04J 14/0221 20130101; H04Q 2011/0035
20130101; H04Q 11/0003 20130101; G02B 6/3512 20130101; G02B 6/3556
20130101; H04Q 2011/0016 20130101; G02B 6/2931 20130101; H04J
14/0212 20130101; G02B 6/29373 20130101; H04Q 11/0005 20130101;
H04J 14/06 20130101 |
International
Class: |
H04Q 11/00 20060101
H04Q011/00; H04J 14/02 20060101 H04J014/02; H04J 14/06 20060101
H04J014/06 |
Claims
1-53. (canceled)
54. An optical signal manipulation system including: a plurality of
ports for carrying a plurality of optical beams to be manipulated,
each optical beam including a plurality of independent wavelength
channels; a polarizing module for polarizing a first subset of
beams from the series of optical beams into a first polarization
state and for polarizing a second subset of beams from the series
of optical beams into a second polarization state orthogonal to the
first polarization state; a wavelength dispersion element for
spatially separating the plurality of independent wavelength
channels of the first and second subsets of beams in the direction
of a first dimension; a separation element for spatially separating
the plurality of wavelength channels of the first and second
subsets of beams in the direction of a second dimension orthogonal
to the first dimension; and a processing device including a series
of independent wavelength processing elements for separately
processing each of the separated wavelengths of the first and
second subsets of beams, with wavelength channels of the first
subset of beams being processed independently of wavelength
channels of the second subset of beams at a location spatially
separated in the second dimension.
55. The system according to claim 54 further comprising a beam
confining module that spatially confines the first subset of beams
with the second subset of beams along a direction of propagation to
a predefined spatial offset in the first dimension.
56. The system according to claim 55 wherein the beam confining
module comprises a polarization beam splitter that reflects the
first subset of beams and transmit the second subset of beams.
57. The system according to claim 55 wherein the beam confining
module comprises a pair of spatially offset and substantially
parallel reflective surfaces, a first reflective surface of the
pair being positioned to reflect the first subset of beams and a
second reflective surface of the pair being positioned to reflect
the second subset of beams.
58. The system according to claim 54 wherein the separation element
comprises a polarization beam splitter that reflects beams from a
first source having a first polarization state and that transmits
beams from a second source having a second polarization state.
59. The system according to claim 58 further comprising a
reflective element that reflects beams from the second source onto
the processing device.
60. The system according to claim 58 wherein the separation element
includes a half-wave plate positioned so that it rotates the beams
of the first source into the same polarization state as the beams
of the second source.
61. The system according to claim 58 further comprising a
birefringent wedge for angularly dispersing the beams from each
source prior to incidence onto the polarization beam splitter.
62. An optical switching device for processing optical beams with
more than one independent wavelength channel, the device
comprising: one or more input ports for inputting optical beams in
a forward direction of propagation; a switching module for
reflecting beams propagating in the forward direction and
selectively switching the optical beams along predetermined paths
in a return direction of propagation; one or more output ports for
receiving predetermined optical beams propagating in the return
direction; and a port selecting module for selectively directing
the beams such that predetermined ones of the beams propagating in
the return direction propagate along trajectories out of alignment
with the input ports.
63. The optical switching device according to claim 62 wherein the
port selecting module includes one or more polarizing elements that
polarizes the optical beams into a predetermined polarization
state.
64. The optical switching device according to claim 63 wherein the
port selecting module further comprises: a polarization separation
element that spatially separates an optical beam into two
orthogonal polarization components; a polarization rotation element
that selectively rotates the polarization components with respect
to each other; and an optical power element that focuses the
polarization components together in a direction of spatial
separation.
65. The optical switching device according to claim 64 wherein the
polarization separation element includes a birefringent walk-off
crystal element.
66. The optical switching device according to claim 64 wherein the
input and output ports are disposed in an array extending in a
first dimension where the spatial separation of polarization
components is in a second dimension perpendicular to the first
dimension.
67. The optical switching device according to claim 64 wherein the
polarization rotation element includes a Faraday rotator that
applies a 45.degree. rotation to a polarization component.
68. The optical switching device according to claim 67 wherein the
polarization rotation element comprises a half-wave plate element
that rotates a first polarization component in a forward
propagation direction and that rotate a second polarization
component in a return propagation direction.
69. The optical switching device according to claim 68 wherein the
first polarization component and the second polarization component
are the same component.
70. The optical switching device according to claim 66 further
comprising a second polarization separation element that spatially
separates the two orthogonal polarization components in the first
dimension.
71. The optical switching device according to claim 64 wherein the
polarization rotation element is reconfigurable so as to
selectivity define specific ports as being either an input port or
an output port.
72. The optical switching device according to claim 71 wherein the
polarization rotation element includes a transmissive liquid
crystal element device including a plurality of individually
drivable electro-optic cells.
73. The optical switching device according to claim 72 wherein the
electro-optic cells are selectively electrically drivable between
two discrete polarization rotation states, wherein one state is
configured to couple an optical beam into a predetermined output
port and another state is configured to couple an optical beam away
from a predetermined output port.
74. The optical switching device according to claim 66 further
comprises a beam shifting element for selectively applying a
spatial shift to the optical beams in the first dimension based on
the spatial position and polarization of the beams.
75. A wavelength manipulation device for independently manipulating
optical beams from a first and second source, the optical beams
having orthogonal polarizations and including a plurality of
individual wavelength channels, the device comprising: an
electrically controllable directing element for simultaneously
selectively spatially directing the optical beams from both the
first and second sources along predefined trajectories onto a
processing device; and a processing device including an array of
independently controllable processing elements for separately and
independently processing the beams from the first and second
sources.
76. A device according to claim 75 wherein the electrically
controllable directing element comprises a MEMS mirror.
77. The device according to claim 75 wherein the electrically
controllable directing element varies the predefined trajectories
in response to data indicative of a local temperature of the
device.
78. The device according to claim 75 wherein the electrically
controllable directing element is responsive to a detected optical
reference signal.
Description
CROSS REFERENCE TO RELATED APPLICATION SECTION
[0001] The present application claims priority to U.S. Provisional
Patent Application No. 61/673,580, filed on Jul. 19, 2012, entitled
"Polarization Diverse Wavelength Selective Switch." The entire
content of U.S. Provisional Patent Application No. 61/673,580 is
herein incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to an optical switching device
and in particular to a wavelength selective switch (WSS)
implementing polarization manipulation optics. While some
embodiments will be described herein with particular reference to
that application, it will be appreciated that the invention is not
limited to such a field of use, and is applicable in broader
contexts.
BACKGROUND
[0003] Any discussion of the background art throughout the
specification should in no way be considered as an admission that
such art is widely known or forms part of common general knowledge
in the field.
[0004] With the development of more complex optical networks, the
capabilities of optical switching devices are evolving. Switches,
such as wavelength selective switches (WSS) are incorporating more
input and output ports to manage the increased system demand.
Further, WSS devices can be configured to independently route
signals from two sources within a single device. In such a
configuration, a single WSS device essentially operates as two
separate devices. An example of a dual source or "twin" device is
described in U.S. Pat. No. 7,397,980 to Frisken, entitled
"Dual-source optical wavelength processor" and assigned to Finisar
Corporation.
[0005] However, constraints on the size of the device generally
place limits on the number and location of possible ports and the
functionality of the device. Further, with increased port numbers,
directivity issues become more prominent as the number of possible
switching states gives rise to undesired connectivity between pairs
of ports. That is, establishing a particular link between two ports
simultaneously establishes links between other pairs of ports. Such
undesired links become more common as the number of ports in a
switching device increases and also as the number of network
switching points increases, particularly in networks utilizing
bidirectional dual source WSS devices. Undesired links between
ports can establish or enhance multipath interference in the beam
along the path and can also cause instability in source
transmitters such as lasers.
[0006] In the case of dual source devices, the independent routing
of a dual source device can be difficult to maintain as the setting
up of switching states of a first optical source may simultaneously
couple signals to ports intended for the second source. In both
single source and dual source devices, this connectivity issue is
often addressed by setting up of isolator arrays on the input
ports. However, this adds to optical loss, size and cost.
[0007] There is a need for improved port isolation in optical
switching devices.
[0008] In current dual source WSS devices beams from the two
sources are propagated separately in space through the optical
system. This separation of beams requires larger sized optical
components and gives rise to increased physical device size
compared to conventional single source devices. Increase component
and device size typically leads to increase manufacturing cost.
[0009] There is also a need for improved dual source WSS
devices.
SUMMARY OF THE INVENTION
[0010] It is an object of the invention, in its preferred form to
provide an improved or alternative WSS device.
[0011] In accordance with a first aspect of the present invention
there is provided an optical switching device for processing
optical beams with more than one independent wavelength channel,
including: [0012] one or more input ports for inputting optical
beams in a forward direction of propagation; [0013] a switching
module for reflecting beams propagating in the forward direction
and selectively switching the optical beams along predetermined
paths in a return direction of propagation; [0014] one or more
output ports for receiving predetermined optical beams propagating
in the return direction; and [0015] a port selecting module for
selectively directing the beams such that predetermined ones of the
beams propagating in the return direction propagate along
trajectories out of alignment with the input ports.
[0016] The port selecting module preferably includes one or more
polarizing elements for polarizing the optical beams into a
predetermined polarization state. The port selecting module
preferably further includes: [0017] a polarization separation
element for spatially separating an optical beam into two
orthogonal polarization components; and [0018] a polarization
rotation element for selectively rotating the polarization
components with respect to each other.
[0019] The polarization separation element preferably includes a
birefringent walk-off crystal element.
[0020] The input and output ports are preferably disposed in an
array extending in a first dimension and the spatial separation of
polarization components is in a second dimension perpendicular to
the first dimension.
[0021] The polarization rotation element preferably includes a
Faraday rotator configured to apply a 45.degree. rotation to a
polarization component. The polarization rotation element
preferably further includes a half-wave plate element configured to
rotate a first polarization component in a forward propagation
direction and to rotate a second polarization component in a return
propagation direction. The first polarization component and the
second polarization component are preferably the same
component.
[0022] The optical switching device preferably includes an optical
power element for focusing the polarization components together.
The optical switching device preferably includes a second
polarization separation element for spatially separating the two
orthogonal polarization components in the first dimension.
[0023] The polarization rotation element is preferably
reconfigurable to allow selectivity to define specific ports as
being either an input port or an output port. In one embodiment,
the polarization rotation element preferably includes a
transmissive liquid crystal element device including a plurality of
individually drivable electro-optic cells. Preferably, the
electro-optic cells are selectively electrically drivable between
two discrete phase states, one phase state configured to couple an
optical beam into a predetermined output port and one phase state
configured to couple an optical beam away from a predetermined
output port.
[0024] The optical switching device preferably includes three input
ports and one output port.
[0025] The optical switching device preferably includes a
dispersive element for spatially dispersing the optical beams into
a plurality of wavelength channels for independent selective
switching of the channels by the switching module.
[0026] The optical switching device preferably includes a beam
shifting element for selectively applying a spatial shift to the
optical beams in the first dimension based on the spatial position
and polarization of the beams. In one embodiment, the spatial shift
is preferably 125 .mu.m.
[0027] In accordance with a second aspect of the present invention,
there is provided an optical switching method, including: [0028]
defining one or more input ports for inputting optical beams in a
forward direction of propagation; [0029] reflecting beams
propagating in the forward direction and selectively switching the
optical beams along predetermined paths in a return direction of
propagation; [0030] defining one or more output ports for receiving
predetermined optical beams propagating in the return direction;
and [0031] selectively directing the beams such that beams
propagating in the return direction propagate along trajectories
out of alignment with the input ports.
[0032] In accordance with a third aspect of the present invention,
there is provided an optical signal manipulation system including:
[0033] a plurality of ports for carrying a plurality of optical
beams to be manipulated, each optical beam including a plurality of
independent wavelength channels; [0034] a polarizing module for
polarizing a first group of beams from the series of optical beams
into a first polarization state and for polarizing a second group
of beams from the series of optical beams into a second
polarization state orthogonal to the first state; [0035] a
wavelength dispersion element for spatially separating the
plurality of wavelength channels of the first and second groups in
the direction of a first dimension; and [0036] a wavelength
manipulation module having: [0037] a separation element for
spatially separating the plurality of wavelength channels of the
first and second groups in the direction of a second dimension
orthogonal to the first dimension; and [0038] a processing device
including a series of independent wavelength processing elements
for separately processing each of the separated wavelengths of the
first and second group, with wavelength channels of the first group
being processed independently of wavelength channels of the second
group at a location spatially separated in the second
dimension.
[0039] The optical signal manipulation system preferably includes a
beam confining module for spatially confining the first group of
beams with the second group of beams along a direction of
propagation to a predefined spatial offset in the first dimension.
The spatial offset in the first dimension is preferably 300 .mu.m.
The beam confining module preferably includes a polarization beam
splitter configured to reflect the first group of beams and
transmit the second group of beams.
[0040] The beam confining module preferably includes a pair of
spatially offset substantially parallel reflective surfaces, a
first reflective surface of the pair being positioned to reflect
the first group of beams and a second reflective surface of the
pair being positioned to reflect the second group of beams. The
pair of reflective surfaces preferably are both surfaces of a
single prism element.
[0041] In one embodiment, the ports are preferably divided into a
first group of ports for carrying the first group of beams and a
second group of ports for carrying the second group of beams, and
wherein the two groups of ports are disposed parallel to each
other. In another embodiment, the ports are divided into a first
group of ports for carrying the first group of beams and a second
group of ports for carrying the second group of beams, and wherein
the two groups of ports are disposed at an angle relative to each
other.
[0042] In accordance with a fourth aspect of the present invention,
there is provided a wavelength manipulation device for
independently manipulating optical beams from a first and second
source, the optical beams having orthogonal polarizations and
including a plurality of individual wavelength channels, the device
including: [0043] a separation element for spatially separating the
optical beams from the first and second sources by polarization for
incidence onto a processing device; and [0044] a processing device
including an array of independently controllable processing
elements for separately and independently processing the beams from
the first and second sources.
[0045] The beams are preferably incident substantially normally
onto the processing device. The separating element is preferably
configured to receive the beams in a first plane and project them
onto the processing device in a second plane relative to the first
plane. The second plane is preferably substantially normal to the
first plane.
[0046] The separation element preferably includes a polarization
beam splitter configured to reflect beams from a first source
having a first polarization state and to transmit beams from a
second source having a second polarization state.
[0047] The wavelength manipulation device preferably includes a
reflective element configured to reflect beams from the second
source onto the processing device.
[0048] The separation element preferably includes a half-wave plate
positioned for rotating the beams of the first source into the same
polarization state as the beams of the second source.
[0049] The wavelength manipulation device preferably includes a
birefringent wedge for angularly dispersing the beams from each
source prior to incidence onto the polarization beam splitter.
[0050] In accordance with a fifth aspect of the present invention,
there is provided an optical manipulation device, including: [0051]
a polarization separation element for spatially separating at least
one input optical beam into first and second orthogonal
polarization components; [0052] a polarization rotation element for
rotating the polarization orientation of the first polarization
component into the same orientation as the second polarization
component; [0053] at least one directing element for directing the
first and second polarization components along substantially
parallel but spatially separated output trajectories.
[0054] The polarization separation element is preferably a
polarization beam splitter. The polarization rotation element is a
preferably reflective half-wave plate. The reflective half-wave
plate preferably defines, in part, the at least one directing
element for directing the first polarization component. The at
least one directing element preferably includes an angled mirror
for directing the second polarization component.
[0055] The optical manipulation device preferably includes a second
polarization separation element for angularly separating the at
least one input beam into orthogonal polarization components. The
second polarization separation element is preferably a birefringent
wedge. The second polarization separation element preferably
angularly separates the orthogonal polarization components in a
dimension perpendicular to the spatial separation performed by the
first polarization separation element.
[0056] The output trajectories of the polarization components are
preferably substantially perpendicular to the trajectory of the at
least one input optical beam.
[0057] The optical manipulation element preferably includes a
processing device having an array of independently controllable
processing elements for separately and independently processing the
first and second polarization components.
[0058] In accordance with a sixth aspect of the present invention,
there is provided a wavelength manipulation device for
independently manipulating optical beams from a first and second
source, the optical beams having orthogonal polarizations and
including a plurality of individual wavelength channels, the device
including: [0059] an electrically controllable directing element
for simultaneously selectively spatially directing the optical
beams from both the first and second sources along predefined
trajectories onto a processing device; and [0060] a processing
device including an array of independently controllable processing
elements for separately and independently processing the beams from
the first and second sources.
[0061] The electrically controllable directing element preferably
includes a MEMS mirror. In one embodiment, the electrically
controllable directing element is preferably configured to vary the
predefined trajectories in response to data indicative of a local
temperature of the device. In another embodiment, the electrically
controllable directing element is preferably responsive to a
detected optical reference signal.
[0062] In accordance with a seventh aspect of the present
invention, there is provided an optical manipulation method,
including: [0063] receiving one or more polarized optical beams at
a position along a first axis perpendicular to a propagation
direction of the one or more beams; [0064] applying a spatial shift
to the one or more beams along the first axis based on the
polarization and position of the one or more beams along the first
axis; and [0065] simultaneously compensating the one or more beams
for one or more aberrations based on the position of the one or
more beams along the first axis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0066] Preferred embodiments of the disclosure will now be
described, by way of example only, with reference to the
accompanying drawings in which:
[0067] FIG. 1 is a schematic perspective view of a WSS device
according to a first embodiment;
[0068] FIG. 2 is a schematic exploded perspective view of a port
selecting module for an optical switch, showing polarization states
of optical beams throughout the module;
[0069] FIG. 3 is a schematic illustration of a symmetric
polarization loop established in the WSS of FIG. 1;
[0070] FIG. 4 is a schematic plan view of the selecting module of
FIG. 2 showing the trajectory and polarization states of an optical
beam passing through the module between an input port and an output
port;
[0071] FIG. 5 is a schematic plan view of the selecting module of
FIG. 2 showing the trajectory and polarization states of an optical
beam passing through the module between two input ports;
[0072] FIG. 6 is a schematic perspective view of a half-wave plate
in the form of a reconfigurable transmissive liquid crystal
device;
[0073] FIG. 7 is a schematic illustration of a symmetric
polarization loop established in the WSS of FIG. 1, showing
diffractive effects at the LCOS device;
[0074] FIG. 8 is a schematic exploded perspective view of a port
selecting module for an optical switch according to a second
embodiment, showing polarization states of optical beams throughout
the module;
[0075] FIG. 9 is a plan view of a beam shifting element used in the
port selecting module of FIG. 8, illustrating beam components in
the input direction;
[0076] FIG. 10 is a plan view of a beam shifting element used in
the port selecting module of FIG. 8, illustrating `wanted` beam
components in the return direction;
[0077] FIG. 11 is a plan view of a beam shifting element used in
the port selecting module of FIG. 8, illustrating `unwanted` beam
components in the return direction;
[0078] FIG. 12 is a schematic perspective view of a WSS device
according to a second embodiment;
[0079] FIG. 13 is a sectional front view of an LCOS device showing
the relative positioning of wavelength channels between two input
sources;
[0080] FIG. 14 is a schematic plan view of a beam confining module
according to an embodiment;
[0081] FIG. 15 is a schematic plan view of a beam confining module
according to another embodiment;
[0082] FIG. 16 is a side view of a separation element used in the
WSS device of FIG. 12;
[0083] FIG. 17 is a schematic illustration of a WSS device
incorporating a beam correction module with electrically
controllable MEMS mirror for providing simultaneous active beam
control to two independent optical devices;
[0084] FIG. 18 is schematic plan view of the beam correction module
used in the WSS device of FIG. 17;
[0085] FIG. 19 is a sectional side view of an alternate beam
correction module wherein the MEMS mirror is mounted co-planar with
the substrate;
[0086] FIG. 20 is a plan view of a turning mirror used in an
embodiment of the beam correction module; and
[0087] FIG. 21 is a schematic illustration of the evolution of the
polarization state of the beam through a turning mirror and a
quarter-wave plate.
DETAILED DESCRIPTION
[0088] The techniques and improvements included in the present
application will be described in the context of an optical
wavelength selective switch (WSS) for switching wavelength channels
contained within wavelength division multiplexed (WDM) optical
signals. For example, the optical signals may comprise dense
wavelength division multiplexed signals including a plurality of
individual wavelength channels equally spectrally separated by 50
GHz. However, it will be appreciated that these techniques and
improvements are able to be implemented in other types of optical
switching and manipulation devices.
General Operation of the WSS Optical Switch
[0089] Referring initially to FIG. 1, there is illustrated an
exemplary WSS optical switching device 1 configured for switching
input optical beams from three input optical fiber ports 3, 5 and 7
to an output optical fiber port 9. Ports 3, 5, 7 and 9 are adapted
for releasable connection to respective optical fibers (not shown).
The optical beams are indicative of WDM optical signals, as
mentioned above. On a broad functional level, device 1 performs a
similar switching function to that described in U.S. Pat. No.
7,397,980 to Frisken, entitled "Dual-source optical wavelength
processor" and assigned to Finisar Corporation, the contents of
which are incorporated herein by way of cross-reference. The
optical beams propagate from input ports 3, 5 and 7 in a forward
direction and are reflected from a liquid crystal on silicon (LCOS)
device 11 (described below) in a return direction to output port
9.
[0090] Ports 3, 5, 7 and 9 are equally spaced apart along a first
axis (x-axis) by a distance of about 250 .mu.m so as to accommodate
optical fibers disposed in a fiber v-groove array, which are also
equally spaced by 250 .mu.m. In other embodiments, ports 3, 5, 7
and 9 are equally spaced apart in the x-axis by other distances. In
further embodiments, ports 3, 5, 7 and 9 are not equally spaced
apart in the x-axis. In some embodiments, ports 3, 5 and 7 include
micro-lenses for controlling the divergence and profile of the
beams exiting or entering the ports from connected optical fibers.
In one embodiment, these micro-lenses are mounted to the optical
fibers themselves. In another embodiment, the micro-lenses are
disposed in an array adjacent the ports in the z-axis or
propagation direction. In a further embodiment, micro-lenses are
included in a fiber v-groove array disposed along the x-axis.
[0091] Device 1 includes a wavelength dispersive grism element 13
for spatially dispersing the individual wavelength channels from an
input optical beam in the direction of a second axis (y-axis).
Grism element 13 operates in similar a manner to that described in
U.S. Pat. No. 7,397,980. That is, to spatially separate the
constituent wavelength channels contained within each optical beam
in the y-axis according to wavelength. In various embodiments,
grism 13 may be formed of suitable materials to provide a low
polarization dependent loss or a reduced polarization sensitivity
to further enhance the polarization diversity of device 1.
[0092] A collimating lens 15 is positioned adjacent to grism 13
such that the optical beams traverse the lens both prior to
incidence onto grism 13 and after reflection from the grism. This
double pass of lens 15 acts to collimate beams in the x-axis.
Similarly, in propagating between input ports 3, 5 and 7 and LCOS
device 11, the beams reflect twice off a cylindrical mirror 17.
Mirror 17 has appropriate curvature in the y-axis such that each
dispersed channel is focused in the y-axis onto the LCOS device. In
another embodiment (not shown), focusing in the y-axis is provided
by two cylindrical mirrors, each having substantially the same
radius of curvature in the y-axis. In further embodiments, the
cylindrical mirrors have different radii of curvature.
[0093] The dispersed wavelength channels are incident onto LCOS
device 11, which acts as a reflective optical manipulation device
to independently steer each channel in the x-axis. At the device
level, LCOS device 11 operates in a similar manner to that
described in U.S. Pat. No. 7,092,599 to Frisken, entitled
"Wavelength manipulation system and method" and assigned to Finisar
Corporation, the contents of which are incorporated herein by way
of cross-reference. However, due to the flexibility available in
these devices, LCOS device 11 is also able to be driven in other
arrangements, such as in a dual source mode of operation, as
described below.
[0094] LCOS device 11 includes a two-dimensional array of
substantially square-shaped cells 19 formed in a layer of liquid
crystal material. In an exemplary embodiment, device 11 includes an
array of 1280 by 768 cells. Each cell is independently electrically
drivable to impose a relative phase shift to a local region of an
incident optical beam. The cells are able to be driven at different
relative levels to define a phase profile which manipulates the
optical wavefront to selectively steer the beam.
[0095] LCOS device 11 steers the wavelength channels at certain
angles along predetermined paths in a return direction such that
some wavelengths are coupled to output port 9. Other wavelength
channels are steered at other angles that couple them away from
output port 9, thereby dropping them from the system.
[0096] It will be appreciated that in other embodiments, device 1
includes different numbers of input and output ports and is
configured to simultaneously couple beams between different input
and output ports. In some embodiments, grism 13 is replaced with a
diffraction grating or other diffractive device. In some
embodiments, LCOS device 11 is replaced with a
micro-electromechanical mirror (MEMS) based optical manipulation
device or other type of optical manipulation device.
[0097] Referring still to FIG. 1, the input optical beams are
transmitted through a port selecting module 21, which provides
polarization diversity to device 1 and provides capability to
restrict optical beams returning from LCOS device 11 from being
coupled back into input ports 3, 5 and 7. The operation of port
selecting module 21 is described below.
Port Selectivity in the WSS
[0098] Referring to FIG. 2, there is illustrated a schematic
exploded side view of port selecting module 21. In traversing
module 21, example optical beam 22 initially passes through a
polarization separation element in the form of a birefringent
walk-off crystal element 23 for spatially separating in the y-axis
an optical beam into two orthogonal polarization components. A
first polarization component 25 of each optical beam is designated
by solid circles. A second polarization component 27 of each
optical beam is designated by dashed circles. The illustrated
orientations of the polarization states is exemplary only and it
will be appreciated by the skilled person that arbitrary
orthogonally polarization states can be generated.
[0099] Walk-off element 23 is formed of a birefringent crystalline
material that has a material optic axis disposed at an angle
relative to the surface normal. Beam components polarized parallel
to the crystalline optic axis (parallel to component 27 in the
illustrated embodiment) are refracted or walked-off from the
original direction of propagation by an amount dependent on the
refractive index and thickness of the element. Beam components
polarized perpendicular to the crystalline optic axis are
unaffected by the material. In preferred embodiments, walk-off
element 23 has a thickness in the order of millimeters and provides
a spatial separation of polarization components in the order of
microns. In the illustrated embodiment, the beams are separated
into components having vertical and horizontal orientations and
propagating parallel at the output of element 23. However, in other
embodiments, element 23 is able to be configured to split the beams
into pairs of orthogonal polarization components having any
orientation.
[0100] In another embodiment, walk-off element 23 is replaced with
a birefringent wedge, which angularly separates two orthogonal
components by an angle determined by the refractive index and angle
of the wedge. In embodiments incorporating birefringent wedges, it
is sometimes necessary to angle input ports with respect to output
ports or vise-versa in the direction of polarization separation
(y-axis).
[0101] The beam components output from walk-off element 23 are then
passed through a half-wave plate element 29. This element includes
birefringent regions 31 and 33 which impose a 180.degree. or 7
radians phase shift between constituent sub-components of that
particular polarization component to rotate the polarization by
90.degree.. Region 31 of element 29 is configured to rotate
component 27 propagating in the forward direction by 90.degree..
Component 25 is passed through element 29 without rotation. After
passing through element 29, both components propagate in the
vertical orientation, as illustrated in FIG. 2. Exemplary
birefringent materials that are used to form regions 31 and 33
include calcite, tourmaline, quartz, sodium nitrate, lithium
niobate and rutile.
[0102] Region 33 of element 29 is configured to rotate component 27
again in the return direction by 90.degree.. However, it will be
appreciated that, in other embodiments, the location of regions 31
and 33 can be varied in the y-axis to rotate the other polarization
components of each beam. These other embodiments are able to
provide the same functionality as that illustrated in FIG. 2. The
general requirement is that the location of regions 31 and 33 must
be oppositely disposed in the y-axis about the vertical center
(x-axis) of element 29 such that the same components rotated in the
forward direction are again rotated in the return direction. That
is, region 31 may be disposed to the left of region 33 in the
y-axis or, alternatively, region 31 may be disposed to the right of
region 33 in the y-axis.
[0103] The specific location of regions in the x-axis defines which
ports will operate as input ports and which ports will operate as
output ports. By way of example, in FIG. 2, beams from the input
ports 3, 5 and 7 are transmitted through region 31 of element 29
and beams returned to output port 9 are transmitted through region
33, which is disposed opposite to region 31 in the y-axis.
[0104] It will be appreciated that in other embodiments, the
location of regions 31 and 33 differ and define devices having
different arrangements of input and output ports. Further, as will
be described below, in one embodiment, element 29 is reconfigurable
to define different positions for regions 31 and 33 for beams
between different arrangements of input and output ports.
[0105] In one embodiment, element 29 is at least partially formed
from a non-birefringent and substantially transparent substrate on
which birefringent materials are mounted to define birefringent
regions such as regions 31 and 33. In some embodiments, the
birefringent materials are able to be moved and mounted or adhered
to different locations of the substrate to provide flexibility to
reconfigure which ports in device 1 are to operate as input ports
and which ports are to operate as output ports. In one particular
embodiment, the substrate is formed of glass. In another
embodiment, regions 31 and 33 define separate half-wave plate
elements, which are able to be positioned at relative positions
along the optical z-axis in alignment with the corresponding beam
components.
[0106] At the output of element 29 in the forward propagation
direction, components 25 and 27 have a common vertical orientation.
The components 25 and 27 are then passed through a Faraday rotator
35 configured to apply a 45.degree. rotation to each polarization
component 25 and 27. Rotator 35 is a non-reciprocal element which
applies the same polarization rotation to beams independent of
propagation direction through the element. As such, in the reverse
direction of propagation, rotator 35 again applies a 45.degree.
rotation to each component 25 and 27, as illustrated in FIG. 2. In
the illustrated embodiment, at the output of the rotator 35 in the
forward direction, each polarization component 25 and 27 has a
+45.degree. orientation. In other embodiments, the polarization
components 25 and 27 have other orientations, depending on the
particular configuration of optical separation and rotation
elements.
[0107] In many cases the optical isolation that is achieved in this
operation can be advantageous though in some cases this will impose
a limitation. If a single polarization is imposed to a beam (either
intentionally or because of single polarization operation of any
component) then the wavelength switch function will be
nonreciprocal (isolating) however if no polarizing element is
imposed within the switching train then the overall device function
will remain reciprocal. In some embodiments, isolation is
established between forward and return path through a spatial
offset. An exemplary embodiment utilizing this isolation is
described below in relation to FIG. 8. Another embodiment providing
improved isolation involves establishing a retro reflection point
at the switching matrix. In these embodiments, spatial diversity is
able to be used to achieve the enhanced directivity rather than
polarization isolation.
[0108] After propagation through Faraday rotator 35 in the forward
direction, the polarization components are passed through a
cylindrical lens 37 having optical power in the y-axis. Lens 37
angularly converges the polarization components together at focal
plane 39, which defines a first point of symmetry in device 1 of
FIG. 1. Referring again to FIG. 1, the polarization components
propagate through device 1 separately and are recombined at the
LCOS device 11, which defines a second point of symmetry in device
1. Between plane 39 and LCOS device 11, a symmetric polarization
loop is established, as illustrated schematically in FIG. 3.
[0109] Referring to FIG. 3, along the loop, one polarization
component propagates clockwise, while the orthogonal component
propagates anti-clockwise. Module 21 rotates both polarization
components into a common orientation (vertical in the illustrated
embodiment) such that polarization dependent effects in the system
are equalized. At focal plane 39, these two polarization components
are spatially confined, as shown in FIG. 3. At the LCOS device 11,
both components are again confined so that they can be
simultaneously manipulated by common cells of device 11. In other
embodiments, the polarization components are rotated into
orientations other than vertical. As the LCOS device is
polarization dependent, the polarization components are preferably
rotated into alignment with the polarization axis of the LCOS
device. In other embodiments utilizing spatial light modulators
other than LCOS devices, this alignment of the polarization
components with a predefined axis may not be required.
[0110] In other embodiments, module 21 is placed at other locations
in the optical system and performs substantially the same function
as described above. The general requirement is that the
polarization equalization performed by module 21 occurs before the
optical beams reach polarization dependent optical elements such
grism 13 and LCOS 11 of FIG. 1.
[0111] When the polarization components return to port selecting
module 21 of FIG. 2, they converge at plane 39 and are collimated
in passing back through lens 37. The components return through
Faraday rotator 35, half-wave plate element 29 and walk-off element
23 in a similar manner to that described in relation to the forward
direction. Walk-off element 23 either recombines the components for
coupling to output port 9 or couples them out of alignment with the
ports to attenuate them. The selection of which beam to couple to
the output port is made by the LCOS device, which is selectively
driven to apply a predetermined switching angle to the beams in the
x-axis. In conventional WSS devices, the establishment of a
particular switching state from one input port to an output port
simultaneously couples beams between other ports that are symmetric
about that switching angle. By way of example, in device 1
illustrated in FIG. 1, switching a beam from input port 3 to output
port 9 will simultaneously switch beams from input port 5 to input
port 7, potentially giving rise to multipath interference in
signals transmitted between ports 5 and 7 and instabilities in
laser sources connected to those ports.
[0112] The particular configuration of elements in port selecting
module 21 acts to reduce or minimize this undesired cross coupling
between input ports through a selective process of coupling
polarization states. This process will now be described with
reference to FIGS. 4 and 5, which illustrate schematic plan views
of port selecting module 21 of FIG. 2. Referring initially to FIG.
4, there is illustrated a schematic plan view of port selecting
module 21 illustrating the spatial evolution of beams propagating
between input port 3 and output port 9. As in FIG. 2, first
polarization component 25 is indicated by solid circles and the
orthogonal component 27 is indicated by dashed circles.
[0113] As described above, in the forward direction, input port 3
projects optical beam 22 through walk-off element 23, half-wave
plate element 29 and Faraday rotator 35 to spatially separate the
two orthogonal polarization components and rotate them into the
same orientation. In this forward direction, component 25 remains
aligned with input port 3 in the y-axis and component 27 is
refracted out of alignment with port 3 by walk-off element 23.
[0114] In the return direction, component 27 is axially aligned
with output port 9 and component 25 is out of alignment with output
port 9. In this return direction, both components pass through
rotator 35 and are rotated 45.degree. such that they are both
oriented horizontally. Second component 27 is passed through
birefringent region 33 to rotate it into a vertical orientation.
Component 25 passes through element 29 without a rotation in
orientation and reaches walk-off element 23 in a horizontal
orientation and offset from output port 9. In traversing walk-off
element 23, component 25 experiences walk-off and is refracted
towards port 9 due to its alignment with the preferred axis of
element 23 horizontal in this embodiment). After passing walk-off
element 23, component 25 is axially aligned with port 9 in the
y-axis. Component 27 is in the vertical orientation and passes
directly through walk-off element 23 without refraction, thereby
remaining axially aligned with output port 9. Therefore, both
components 25 and 27 are recombined and coupled efficiently into
output port 9. Similar coupling occurs between ports 5 and 9, and
between ports 7 and 9 and the optical loss incurred by each
polarization state is substantially equal.
[0115] Due to symmetry in the optical system, to perform the
switching described above, LCOS device 11 also simultaneously sets
up a switching path between input ports 5 and 7 of FIG. 2.
Referring now to FIG. 5, there is illustrated a schematic plan view
of port selecting module 21 illustrating the spatial evolution of
beams propagating between input port 5 and input port 7. As with
FIGS. 2 and 4, polarization component 25 is indicated by solid
circles and the orthogonal component 27 is indicated by dashed
circles.
[0116] Propagation in the forward direction from port 5 is
identical to that described above in relation to FIG. 4. However,
in the return direction, the system is asymmetric due to the
positioning of region 31 of element 29. Component 25 passes through
region 31 of element 29, experiencing a polarization rotation of
90.degree. into a vertical orientation. Component 27 is unaffected
by element 29 and remains in a horizontal orientation. In passing
through walk-off element 23, component 27 experiences walk-off due
to its polarization alignment with the preferred axis of the
element. This walk-off refracts component 27 out of alignment with
input port 7. Component 25 is not affected by element 23 due to its
anti-alignment with the preferred axis of element 23. Component 25
passes directly through element 23 and remains out of alignment
with input port 7. Therefore, neither component is coupled to input
port 7.
[0117] Comparing FIGS. 4 and 5, it can be seen that the symmetry in
optical path between input port 3 and output port 9 provides
effective coupling of an optical beam between the ports. However,
the asymmetry in optical path between input port 5 and input port 7
restricts the optical beam from coupling between the input ports,
significantly reducing interference effects to optical signals from
the other input ports. The symmetry is controlled by the relative
positioning of the half-wave plate regions 31 and 33 such that both
separated polarization components undergo the same relative changes
in the return direction as in the forward direction. This symmetry
is not present between two input ports, only between input ports
and output port 9.
[0118] As illustrated in the plan views of FIGS. 4 and 5, switching
paths that include a half-wave plate element on opposite sides (in
the y-axis) between the forward and return directions will provide
symmetry and therefore facilitate coupling between an input port
and output port. Conversely, switching paths that include a
half-wave plate element on the same side (in the y-axis) between
the forward and return directions will not provide symmetry and
therefore will not couple beams between the fibers. A symmetric
path defines a switching path and an asymmetric path defines a
non-switching path. From this it can be observed that the choice of
a switching path can be made by the relative positioning of
half-wave plate elements in the y-axis.
[0119] Referring generally to FIGS. 2 to 5, it will be appreciated
that the orientations of the particular polarization components
described above are exemplary only. In other embodiments, module 21
is configured to manipulate polarization components having
different orientations while performing the same functionality.
Specifically, walk-off element 23 is configured to split optical
beams into polarization components other than horizontal and
vertical. Similarly, the relative position of regions 31 and 33 of
element 29 are able to be interchanged while still performing the
same overall polarization manipulation.
[0120] In another embodiment (not shown), module 21 includes a
half-wave plate located between element 29 and lens 37. This
additional half-wave plate is configured to apply a further
arbitrary polarization rotation so as to propagate the beams
through device 1 in a preferred polarization state. In a further
embodiment, element 29 includes an array of half-wave plates that
act to apply different rotations to each of the beams so that they
can arrive at the Faraday rotator at an equal but arbitrary
orientation.
[0121] In one embodiment (not shown) module 21 includes an
additional polarizing element having a polarizing axis oriented
along a preferred axis. In one embodiment this polarizing element
is located between elements 35 and 37. The polarizing element acts
to filter out optical power that has strayed from the desired
orientation so as to improve isolation between polarization
states.
[0122] Referring now to FIG. 6, there is illustrated an alternative
embodiment half-wave plate element in the form of a reconfigurable
transmissive liquid crystal device 41. Device 41 is able to replace
element 29 of FIGS. 2, 4 and 5. Like LCOS device 11 of FIG. 1,
device 41 includes a two-dimensional array of independently
drivable phase manipulating cells configured to impose a relative
phase shift to a local area of an incident optical beam. The cells
are divided into eight regions 43, 45, 47, 49, 51, 53, 55 and 57
that are axially aligned with polarization components of each beam
(walk-off element 23 is omitted from FIG. 6 for simplicity). Within
each region, the cells are selectively electrically driven at one
of two discrete polarization rotation states. A first state
(illustrated by the vertical lines across regions 45, 47, 51 and
55) imposes a relative phase change of 180.degree. or 7 radians
phase shift between constituent sub-components of that particular
polarization component to rotate the polarization by 90.degree..
That is, regions driven in the first state operate as a half-wave
plate. A second state imposes little or no phase change to
essentially pass the polarization component without rotation.
[0123] This selective driving in one of two states allows
selectively defining of specific ports as being configured as
either an input port or an output port. Specifically, defining a
symmetric switching path between two ports allows coupling from one
port to the other. By way of example, device 41 includes four
vertically separated pairs of horizontally adjacent cells. In the
top pair of regions, region 45 is driven in the first state and
region 43 is driven in a second state. The adjacent three pairs of
regions below are driven with an opposite configuration. This sets
up symmetric switching paths between port 3 and any one of ports 5,
7 and 9. Therefore, in one configuration, port 3 can be used as an
input port and ports 5, 7 and 9 as output ports. Alternatively,
ports 5, 7 and 9 can be used as input ports while port 3 is used as
an output port.
[0124] In WSS devices implementing pixilated spatial light
modulators, such as LCOS and MEMS (for example, Texas Instruments
DLP.TM.) devices, undesired diffraction effects are experienced due
to the inherent periodic pattern of the cell structure. The
periodicity of the LCOS surface results in a small amount of
uncontrolled diffraction in addition to the steering applied to
each beam. Referring now to FIG. 7, the symmetric loop path of FIG.
3 is shown with the addition of extra diffraction effects
originating from LCOS device 11, illustrated as dashed arrows at
the LCOS device. While most of the light is directed along the
steering paths 59 and 61, some light is diffracted along other
paths, e.g. path 63. If one of these paths is aligned with the
input path, then a particular polarization component will be
coupled back to the originating input port, resulting in undesired
interference effects which degrade the overall device
performance.
[0125] Standard polarization diversity schemes do not compensate
for these diffractive coupling effects. Referring now to FIG. 8,
there is illustrated another embodiment port selecting module 65
which is capable of compensating for the above described
diffractive coupling effects. Corresponding features of module 21
are designated by the same reference numerals in module 65. Module
65 includes a beam shifting element 67 located between walk-off
crystal 23 and half-wave plate element 29. Module 65 is comprised
of a pair of birefringent wedges, configured to shift one
polarization component with respect to the other in the x-axis.
This shift can be seen more clearly in FIG. 9.
[0126] Turning to FIG. 9, there is illustrated a plan view of beam
shifting element 67 showing propagation of optical beam 22
therethrough. Beam 22 is initially incident onto birefringent wedge
71, which has a crystalline optic axis oriented such that it
refracts or `walks off` component 25 (having a vertical
polarization orientation) downward in the x-axis. Birefringent
wedge 71 is configured such that component 27 (having a horizontal
polarization orientation) passes without any change. The components
then propagate through a second birefringent wedge 73, which has a
crystalline optic axis oriented orthogonal to that of wedge 71. In
propagating through wedge 73, component 25 is walked off upward in
the x-axis. The width of wedge 73 is greater than that of wedge 71
and so component 25 experiences a net refraction upward from its
original trajectory in the x-axis. Through wedge 73, component 27
again remains unrefracted. The choice of wedge angle here is
appropriate to compensate for 1.sup.st order effects of the
polarization dependent switching displacement and may be chosen to
optimize any optical design involving polarization diversity and
multiple ports.
[0127] In effect, element 67 acts to both shift the beams to apply
a 125 .mu.m (or other) beam displacement, and also to apply
correction to beams that are offset from the desired propagation
path. As shown in FIG. 9, the beam shifting function is performed
by the right had side of wedge 73 and the beam correction is
performed by the combination of the left hand side of wedge 73 and
wedge 71. In the beam correction section of the element 67, wedges
71 and 73 have different thicknesses as a function of `x` position.
This means that polarization components that travel at different
heights `x` in device 1 will exit the beam correction section with
a small `x` offset. This subtle variation in the offset as a
function of `x` compensates for system aberrations, and
substantially reduces the overall polarization dependent loss in
device 1.
[0128] It will be appreciated that, in other embodiments, different
methods of shifting one polarization component with respect to the
other. In one exemplary embodiment, element 67 consists only of the
right hand beam shifting side of wedge 73. In another exemplary
embodiment, element 67 includes a beam compensator that refracts
one polarization state at an angle such that, at the output, it is
displaced by about 125 .mu.m from the other polarization
component.
[0129] At the output of element 67, the two components propagate
parallel but with a 125 .mu.m separation in the x-axis, which is
half the spacing of the input and output ports. Referring again to
FIGS. 7 and 8, the system is symmetric such that components that
propagate along the steering paths 59 and 61 are recombined by
element 67 and coupled to the corresponding output port. That is,
component 25, which was shifted by element 67 in the forward
direction, is shifted back to its original position in the x-axis
in the reverse direction. Consequently, components 25 and 27 trace
the same paths back through element 67 on the return path and are
coupled to an output port.
[0130] Conversely, components that are diffracted by the LCOS
device and coupled back along the input path will not be coupled to
an output port. These components propagate back through module 65
where Faraday rotator 35 and half-wave plate element 29 rotate the
polarization components into the orthogonal orientation to that of
the forward direction. Then, upon reaching beam shifting element
67, the component not shifted in the forward direction (component
27 in the illustrated embodiment) is shifted in the return
direction. Similarly, the component shifted in the forward
direction (component 25 in the illustrated embodiment) is not
shifted in the return direction. This situation is illustrated in
FIG. 10, which illustrates a plan view of beam shifting element 67
showing propagation of `unwanted` components of beams 25 and 27 in
the return direction to exemplary output port 9. Here, the position
of each component 25 and 27 is reversed from that of FIG. 9. This
situation results in both components being shifted and offset by
125 .mu.m out of alignment with output port 9, thereby restricting
the components from coupling back to port 9. The situation of
`wanted light` is illustrated in FIG. 11 wherein the components of
beams 25 and 27 that are coupled correctly and symmetrically are
returned in axial alignment with output port 9.
[0131] Therefore, port selecting module 65 restricts back coupling
of optical beams from LCOS device 11, thereby reducing interference
effects and improving device performance.
[0132] The particular shift spacing of 125 .mu.m is chosen as the
input and output fibers are disposed in an array having a fiber
spacing of 250 .mu.m. Therefore, a shift of 125 .mu.m centres the
beams directly between two adjacent ports to minimise
cross-coupling between the ports. In other embodiments utilizing
different fiber port spacings, element 65 is configured to provide
different shifts in the x-axis to accommodate the different spacing
of the ports. In further embodiments element 67 is replaced with a
walk-off crystal which provides an equivalent 125 .mu.m offset in
the x-axis.
[0133] Therefore, the above embodiments provide efficient switching
of optical beams in a WSS device independent of polarization. Beams
being switched between an input port and an output port are coupled
efficiently while inadvertent switching of beams between two input
ports is significantly reduced.
Dual Source Architecture
[0134] As mentioned above, WSS devices can also be configured to
operate as dual source devices wherein two groups of optical beams
are independently coupled between two sources sharing a common
optical system. In known dual source WSS devices, beams from each
source are typically separated in angle and/or space in propagation
through the device to differentiate the two sources. The angular
separation can give rise to aberrations due to angular incidence
onto the diffractive grism and LCOS device. The spatial separation
requires a larger optical setup with larger components, leading to
increased cost.
[0135] Referring now to FIG. 12 there is illustrated a further
embodiment WSS device 75 configured to operate as a dual source
device. Device 75 is constructed to reduce these spatial and
angular separation requirements that are present in conventional
dual source WSS devices. Corresponding features of earlier
described embodiments are indicated by the same reference
numerals.
[0136] Device 75 includes a plurality of ports, which are divided
into a first group 77 for carrying a first group of beams
corresponding to a first optical device (Source A) and a second
group of ports 79 for carrying a second group of beams
corresponding to a second optical device (Source B). Source A is
configured to switch an optical beam 81 from input port 83 to one
or more of a first set of twenty three output ports 85 and 87 (only
two are shown for simplicity). Simultaneously and independently,
Source B is configured to switch an optical beam 89 from input port
91 to one or more of a second set of twenty three output ports 93
and 95 (again, only two are shown for simplicity). In other
embodiments, different numbers of output ports are included in each
device.
[0137] The two groups of ports 77 and 79 are disposed parallel to
each other and transmit beams through corresponding independent
polarizing port selecting modules 97 and 99. In addition to
providing appropriate polarization manipulation, modules 97 and 99
are configured to output beams 81 and 89 in orthogonal polarization
states. In the illustrated embodiment, module 97 outputs a beam
with vertical polarization and module 99 outputs a beam with
horizontal polarization. In one embodiment, modules 97 and 99
comprise port selecting module 21 of FIG. 2, with one module
including elements oriented to provide vertical beam output and the
other module including elements oriented to provide horizontal beam
output. In some embodiments, modules 97 and 99 include port
selecting module 21 of FIG. 2 in addition to other optics. In one
embodiment, modules 97 and 99 include only polarizers oriented with
their axes aligned to output the required polarization orientation.
In further embodiments, other known polarization diversity systems
are implemented in place of, or in addition to modules 97 and
99.
[0138] Referring still to FIG. 12, beams 81 and 89 are passed
through a beam confining module 101 for spatially confining and
realigning beams 81 and 89 along a direction of propagation
(z-axis). This spatial confinement reduces the necessary size of
the optical system and associated coupling elements, and allows for
more efficient use of space on LCOS device 11. At the same time,
the beams are restricted from being spatially overlapped so as to
maintain isolation between the signals from the two sources. In one
embodiment, beams 81 and 89 are confined to a spatial offset of
about 300 .mu.m in the y-axis (wavelength dispersion axis). In
other embodiments, the offsets are chosen by the beam size in the
system, and the required optical isolation between sources A and
B.
[0139] The spatial offset defined by module 101 emerges as an
offset of individual wavelength channels at LCOS device 11.
Referring now to FIG. 13, there is illustrated schematically front
view of a region of LCOS device 11. As shown, the offset of 300
.mu.m defined by confining module 101 provides a 300 .mu.m offset
between corresponding wavelength channels of sources A and B in the
y-axis. This offset provides enhanced isolation between signals
from sources A and B.
[0140] Returning to FIG. 12, confining module 101 includes an
angled mirror 103, which reflects beam 81 and directs it
perpendicularly towards beam 89. Both beams are passed through a
polarization beam splitter (PBS) 105 that is configured to reflect
beam 81. Beam 89, having an orthogonal polarization orientation, is
not affected by the reflective surface and passes directly through
PBS 105. At the output of module 101, beams 81 and 89 propagate
parallel with a predefined spacing.
[0141] It will be appreciated that fiber arrays 77 and 79
corresponding to sources A and B need not be disposed parallel to
each other. By suitable angling of mirror 103, the two sources are
able to be angled with respect to each other. In one exemplary
embodiment, fiber array 77 from Source A is disposed
perpendicularly to fiber array 79 of Source B. This arrangement of
fibers provides simplicity for aligning the beams and enhanced
isolation between the two arrays of fibers. In further embodiments,
fiber arrays 77 and 79 are disposed at angles other than 90.degree.
to each other.
[0142] Referring now to FIG. 14, there is illustrated a second
embodiment beam confining module 108. Module 108 includes a
coupling prism 114.
[0143] To confine the beams from the two sources, beam 81 from
Source A is directed onto a first reflective surface 116 of prism
114 which reflects beam 81 at an angle substantially 90.degree.
from the input direction. Beam 89 from Source B is directed onto a
second reflective surface 118 of prism 114 and is also reflected at
an angle substantially 90.degree. to the input direction.
Reflective surfaces 116 and 118 are disposed substantially parallel
with each other but are spatially separated. The reflection of beam
81 occurs at a longitudinally separate location from that of beam
89 and, upon reflection, this separation translates to a controlled
transverse separation.
[0144] The focal point from focusing lens 37 of module 97 is
located at surface 116 and the focal point from lens 37 of module
99 is located on transmissive surface 112. In other embodiments,
the respective focal points are located elsewhere. In the
illustrated embodiment, the separation between the beams after
module 108 is about 300 .mu.m. However, the exact separation will
be determined by the size of the focal spots at surfaces 116 and
112, and the degree of isolation required between sources A and
B
[0145] Although module 108 outputs confined beams 81 and 89 at a
direction 90.degree. to the input direction, it will be appreciated
that the beams can be reflected off a further 45.degree. mirror
(not shown) to output the beams in the same direction as they were
input. This allows module 108 to be incorporated into device 75 of
FIG. 12 in place of model 101.
[0146] Referring to FIG. 15, there is illustrated a third
embodiment beam confining module 120. Module 120 operates in
substantially the same manner as module 108 of FIG. 12 but is
positioned to output confined parallel beams 81 and 89 at an angle
greater than 90.degree. to the input direction. In various
embodiments, beams 81 and 89 from sources A and B are able to be
confined to arbitrary spatial offsets and directed at arbitrary
angles by varying the orientation and/or distance between surfaces
116 and 118.
[0147] Although, beams 81 and 89 are illustrated as having an input
spatial separation of 3 mm in FIGS. 14 and 15, it will be
appreciated that this spatial separation is exemplary and dependent
upon the particular optical system. In other embodiments, beams 81
and 89 are input from sources A and B with different spacings.
[0148] The fiber ports, polarizing modules 97 and 99 and beam
confining module 101 collectively define a "front end" of WSS
device 75. The "back end" is defined by the switching and
dispersive optics, including the grism 13 and LCOS device 11. In
known dual source WSS systems, beams of separate devices are
spatially separated at the front end for separate processing at the
back end. This front end separation of beams provides lower limits
on the physical size of the source and the optical elements
required to manipulate each beam. In device 75, beams 81 and 89 are
encoded with perpendicular polarization orientations and
transmitted together through the back end of the device.
[0149] Referring still to FIG. 12, device 75 includes grism 13 for
spatially separating from the beams the plurality of wavelength
channels in the y-axis. In various embodiments, grism 13 may be
formed of suitable materials to provide a low polarization
dependent loss or a reduced polarization sensitivity to further
enhance the polarization diversity of device 75. The dispersed
wavelength channels are incident onto a separation element 107 for
spatially separating the wavelength channels in the x-axis by
polarization for incidence onto LCOS device 11. LCOS device 11
includes an array of independently drivable cells for separately
and independently processing each of the separated wavelengths from
devices A and B. As illustrated, wavelength channels of Source A
are processed at a location that is spatially offset in the z-axis
to wavelength channels of Source B (a separation in the x-axis
translates to a translation in the z-axis upon transmission through
separation element 107).
[0150] Referring now to FIG. 16, there is illustrated a side view
of separation element 107, the operation of which will now be
described. Confined beams 81 and 89 are together incident onto a
side of an optional birefringent wedge 109, which angularly
diverges beam 81 from beam 89 with very high polarization
extinction. The diverging beams are then passed through a PBS 111,
which reflects vertically polarized beam 81 and transmits
horizontally polarized beam 89. Beam 81 is reflected vertically
onto a half-wave plate element 113, which reflects beam 81 and
rotates it into a horizontal polarization. On its downward return,
beam 81 passes directly through PBS 111 and is incident
substantially perpendicularly onto a first region 115 of LCOS
device 11. Beam 89 is transmitted onto an angled mirror element
117, which reflects beam 89 substantially perpendicularly onto a
second region 119 of LCOS device 11. The paths travelled by beams
81 and 89 through element 107 are substantially equal.
[0151] Separating element 107 is configured to receive the beams
propagating in the z-axis and project them downward onto LCOS
device 11 in the x-axis. Regions 115 and 119 are offset in the
z-axis such that the independently drivable cells of LCOS device 11
simultaneously and independently route optical beams from devices A
and B through the common optical system. In other embodiments,
separating element 107 does not include wedge 109.
[0152] Returning to FIG. 12, the design of device 75 allows the two
orthogonal beams from separate devices to be transmitted along
spatially confined or overlapping paths and reduces the need for
relatively large physical separation of the beams in the device.
Therefore, the physical dimensions of device 75 are able to be made
smaller than other known dual source WSS devices.
Thermal and Stability Control
[0153] Protection from thermal changes and vibrations to the
switching devices described above is provided in part by the
substrate and enclosure used to support and protect the device. In
some embodiments, the device is mounted on a thick 5 mm substrate
which provides for increased optical stability against effects such
as bending. Further, the device enclosure includes copper shielding
and electronically controllable thermoelectric temperature
controllers mounted to the substrate. All of these temperature and
stability controlling features add to the size of the packaged
device, particularly the device height. There is generally a desire
to minimize the overall package size of an optical device.
[0154] Described below are embodiments that incorporate further
protection from beam misalignment due to temperature changes and
device stability by utilizing active beam control and correction.
Use of active beam correction allows the relaxing of traditional
temperature/stability control features in place of the active
corrective system, thereby allowing designers to remove one or more
controlling features and reducing the overall package size.
[0155] Referring to FIG. 17, there is illustrated a WSS device 135
incorporating an active beam correction module 136. Module 136
includes an electrically controllable MEMS mirror 137 and a
spherical lens 139, and provides simultaneous active beam
correction to two independent optical sources (Source A and Source
B). Functionally, device 135 is substantially similar to device 75
of FIG. 11 with the addition of active beam control. Corresponding
features of earlier described embodiments are indicated by the same
reference numerals in FIG. 17.
[0156] A schematic plan view of the active control system 136 is
illustrated in FIG. 18. MEMs 137 is mounted on one end to a
substrate on which device 135 is mounted so as to be disposed
vertically and extending perpendicularly from the substrate.
Spherical lens 139 is positioned one focal length from both sources
A and B on one side and one focal length from MEMs 137 on the other
side. Lens 139 simultaneously focuses beams from both sources onto
MEMS 137 and collimates the beams returning from MEMS 137 to an
image point 140.
[0157] Referring again to FIG. 17, in terms of relative
positioning, module 136 is located after modules 97 and 99 and
before cylindrical module 101. However, it should be understood
that, in other embodiments, module 136 is located at other points
before module 101 and after sources A and B. In one such
embodiment, module 136 is located within modules 97 and 99, after
elements 35 and before elements 37.
[0158] An angled reflector 141 is positioned to direct beams
90.degree. into the plane of the switching optics, although this
element is not strictly necessary and angles other then 90 degrees
are able to be used. It will be appreciated by the skilled person
that, in other embodiments, different optical configurations of
module 136 can be designed to allow a single source or many sources
for the same single MEMS 137. In another embodiment, MEMS 137 is
replaced with an array of steering elements, each of which is
aligned with a separate source.
[0159] In operation, MEMS 137 is electronically configurable to be
tilted at predetermined angles in one dimension to selectively
adjust the beam trajectories and compensate for misalignments in
device 135. Misalignment of beams in device 135 is caused by, inter
alia, the bending and deformation of optical elements due to
temperature change and mechanical instabilities. MEMS 137 is
controlled by an electronic control system (not shown). In one
embodiment the control system is adapted to receive input from a
temperature sensor and, in response to that input, specify a
particular tilt angle of MEMS 137. In another embodiment, a
reference beam is coupled through one of the sources and received
by an external detector to detect an optimum coupling trajectory.
The data received by the detector is fed to MEMS 137 to specify a
tilt angle to maintain the optimum coupling.
[0160] The MEMS is situated such that each optical beam is
reflected off the mirror at a predefined angle. The MEMS is
controllable to tilt the mirror in one dimension so as to adjust
the trajectory of the beams in one dimension. In another
embodiment, the MEMS is configured to be tiltable in two
dimensions. This selective adjustment allows for correction of beam
trajectories that arise from thermal changes to the optical
elements in the device.
[0161] In device 135, MEMS 137 is mounted vertically onto substrate
123 so as to direct beams propagating across the substrate. In some
embodiments, it is advantageous to mount MEMS 137 horizontally onto
substrate 123 such that MEMS 137 faces vertically upward. In these
embodiments, a slightly different configuration is required, as
described below.
[0162] Referring now to FIG. 19, there is illustrated a schematic
side view of MEMS 137 mounted co-planar with the substrate on which
the device is mounted. In this embodiment, a turning mirror 145 is
used to direct the beam from its substantially horizontal
propagation, vertically downward onto MEMS 137. Before reaching
MEMS 137, the beam is passed through a polarization correcting
quarter-wave plate 147 for rotating the polarization of the beam to
correct for polarization changes induced by turning mirror 145.
After propagation through quarter-wave plate 147, the beam is
reflected off MEMS 137 at an angle depending on the tilt angle of
the device. The reflected beam is passed back through quarter-wave
plate 147 and directed off turning mirror 145 back into the
switching device. The beam passes through this system initially
prior to switching and again after switching has been
performed.
[0163] A plan view of turning mirror 145 is illustrated in FIG. 20.
Optical beams are reflected off mirror 145 twice: once prior to
incidence onto MEMS 137 and once after reflection off MEMS 137.
Each reflection off mirror 145 rotates the polarization of the
beams by an angle .theta., which is dependent upon the incident
angle .alpha. to mirror 145. Quarter-wave plate 147 corrects for
these polarization rotations to ensure the beam output from
correcting module 136 of FIG. 17 has substantially the same
polarization orientation as the input beam.
[0164] Referring to FIG. 21, there is illustrated a schematic
illustration of the evolution of the polarization state of the beam
through turning mirror 145 and quarter-wave plate 147. In
operation, turning mirror 145 rotates the polarization of the beam
by an angle .theta. in one direction depending on the angle of
incidence (a) of the beam onto mirror 145. After two passes through
quarter-wave plate 147, the direction of the rotation of
polarization is flipped to -.theta.. Finally, upon the second pass
of turning mirror 145, the beam undergoes a further polarization
rotation that undoes the initial polarization rotation.
[0165] The operation of the polarization correction is most
efficient when the optical axis of quarter-wave plate 147 is
parallel or perpendicular to the polarization state of the optical
beam. The system described in relation to FIGS. 19 and 20 functions
equivalently and simultaneously for beams originating from both
input sources to device 135 of FIG. 17, even though the incident
beam angles are different and the polarization states of each beam
are orthogonal.
[0166] In a further embodiment (not shown), two MEMS mirrors are
implemented with one MEMS positioned to correct the trajectories
for beams from Source A and the other MEMS positioned to correct
the trajectories for beams from Source B. In other embodiments,
active correcting elements other than MEMs mirrors are utilized. By
way of example, in one embodiment, a LCOS device is used in place
of MEMS mirror 137 and is configured to provide equivalent steering
of the optical beams.
[0167] The active control systems described above are not limited
to the specific embodiments in which it is described. It will be
appreciated that substantially similar active control systems are
able to be implemented into the other embodiments described in this
application, as well as other optical switching devices
generally.
[0168] It will be appreciated that in other embodiments,
combinations of the features described in the above embodiments can
be used. By way of example, in one other embodiment, beams from
independent devices are input perpendicularly, and are also offset
in the y-dimension.
[0169] The person skilled in the art will appreciate that the
principles described above in relation to the dual source
embodiments are also applicable to optical switches incorporating a
single source or more than two independent sources.
CONCLUSIONS
[0170] It will be appreciated that the disclosure above provides
various significant WSS devices. In particular, the embodiments
described herein are adapted to efficiently couple optical beams
between predefined input and output ports while substantially
restricting the internal back-reflection to other input ports. This
improved port selectivity reduces the interference effects
introduced by the WSS device and improves the overall device
performance. Further, with the improved port selectivity, there is
no need to implement isolator arrays on the input ports. This
brings down size, and cost of the WSS, as well as reducing overall
optical loss.
[0171] Some embodiments are reconfigurable, allowing the
interchange of input and output ports. Further, some embodiments
are adapted to provide a dual source WSS architecture providing the
simultaneous and/or bidirectional switching of two optical sources.
In these dual source architectures, the beams from the two sources
are propagated internally with orthogonal polarization states and
processed separately at the switching matrix. This allows the beams
to be transmitted along spatially confined or overlapping optical
paths. This reduces the need for relatively large physical
separation of the beams in the device, thereby reducing the
necessary physical dimensions of the WSS device compared to other
known dual source WSS devices.
[0172] Dual source embodiments of the invention are adapted to
provide increased isolation of the beams at the LCOS device by
applying a spatial offset to the beams at the front-end of the
device. Some embodiments are also adapted to provide active
correction of beam trajectories so as to compensate for beam
misalignments due to thermal and mechanical aberrations in the
WSS.
Interpretation
[0173] Throughout this specification, use of the term "element" is
intended to mean either a single unitary component or a collection
of components that combine to perform a specific function or
purpose.
[0174] Throughout this specification, use of the term "orthogonal"
is used to refer to a 90.degree. difference in orientation when
expressed in a Jones vector format or in a Cartesian coordinate
system. Similarly, reference to a 90.degree. rotation is
interpreted to mean a rotation into an orthogonal state.
[0175] Unless specifically stated otherwise, as apparent from the
following discussions, it is appreciated that throughout the
specification discussions utilizing terms such as "processing,"
"computing," "calculating," "determining", analyzing" or the like,
refer to the action and/or processes of a computer or computing
system, or similar electronic computing device, that manipulate
and/or transform data represented as physical, such as electronic,
quantities into other data similarly represented as physical
quantities.
[0176] In a similar manner, the term "processor" may refer to any
device or portion of a device that processes electronic data, e.g.,
from registers and/or memory to transform that electronic data into
other electronic data that, e.g., may be stored in registers and/or
memory. A "computer" or a "computing machine" or a "computing
platform" may include one or more processors.
[0177] The methodologies described herein are, in one embodiment,
performable by one or more processors that accept computer-readable
(also called machine-readable) code containing a set of
instructions that when executed by one or more of the processors
carry out at least one of the methods described herein. Any
processor capable of executing a set of instructions (sequential or
otherwise) that specify actions to be taken are included. Thus, one
example is a typical processing system that includes one or more
processors. Each processor may include one or more of a CPU, a
graphics processing unit, and a programmable DSP unit. The
processing system further may include a memory subsystem including
main RAM and/or a static RAM, and/or ROM. A bus subsystem may be
included for communicating between the components. The processing
system further may be a distributed processing system with
processors coupled by a network. If the processing system requires
a display, such a display may be included, e.g., a liquid element
display (LCD) or a cathode ray tube (CRT) display. If manual data
entry is required, the processing system also includes an input
device such as one or more of an alphanumeric input unit such as a
keyboard, a pointing control device such as a mouse, and so forth.
The term memory unit as used herein, if clear from the context and
unless explicitly stated otherwise, also encompasses a storage
system such as a disk drive unit. The processing system in some
configurations may include a sound output device, and a network
interface device. The memory subsystem thus includes a
computer-readable carrier medium that carries computer-readable
code (e.g., software) including a set of instructions to cause
performing, when executed by one or more processors, one of more of
the methods described herein. Note that when the method includes
several elements, e.g., several steps, no ordering of such elements
is implied, unless specifically stated. The software may reside in
the hard disk, or may also reside, completely or at least
partially, within the RAM and/or within the processor during
execution thereof by the computer system. Thus, the memory and the
processor also constitute computer-readable carrier medium carrying
computer-readable code.
[0178] Reference throughout this specification to "one embodiment",
"some embodiments" or "an embodiment" means that a particular
feature, structure or characteristic described in connection with
the embodiment is included in at least one embodiment of the
present disclosure. Thus, appearances of the phrases "in one
embodiment", "in some embodiments" or "in an embodiment" in various
places throughout this specification are not necessarily all
referring to the same embodiment. Furthermore, the particular
features, structures or characteristics may be combined in any
suitable manner, as would be apparent to one of ordinary skill in
the art from this disclosure, in one or more embodiments.
[0179] As used herein, unless otherwise specified the use of the
ordinal adjectives "first", "second", "third", etc., to describe a
common object, merely indicate that different instances of like
objects are being referred to, and are not intended to imply that
the objects so described must be in a given sequence, either
temporally, spatially, in ranking, or in any other manner.
[0180] In the claims below and the description herein, any one of
the terms comprising, comprised of or which comprises is an open
term that means including at least the elements/features that
follow, but not excluding others. Thus, the term comprising, when
used in the claims, should not be interpreted as being limitative
to the means or elements or steps listed thereafter. For example,
the scope of the expression a device comprising A and B should not
be limited to devices consisting only of elements A and B. Any one
of the terms including or which includes or that includes as used
herein is also an open term that also means including at least the
elements/features that follow the term, but not excluding others.
Thus, including is synonymous with and means comprising.
[0181] It should be appreciated that in the above description of
exemplary embodiments of the disclosure, various features of the
disclosure are sometimes grouped together in a single embodiment,
Fig., or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claims
require more features than are expressly recited in each claim.
Rather, as the following claims reflect, inventive aspects lie in
less than all features of a single foregoing disclosed embodiment.
Thus, the claims following the Detailed Description are hereby
expressly incorporated into this Detailed Description, with each
claim standing on its own as a separate embodiment of this
disclosure.
[0182] Furthermore, while some embodiments described herein include
some but not other features included in other embodiments,
combinations of features of different embodiments are meant to be
within the scope of the disclosure, and form different embodiments,
as would be understood by those skilled in the art. For example, in
the following claims, any of the claimed embodiments can be used in
any combination.
[0183] In the description provided herein, numerous specific
details are set forth. However, it is understood that embodiments
of the disclosure may be practiced without these specific details.
In other instances, well-known methods, structures and techniques
have not been shown in detail in order not to obscure an
understanding of this description.
[0184] Similarly, it is to be noticed that the term coupled, when
used in the claims, should not be interpreted as being limited to
direct connections only. The terms "coupled" and "connected," along
with their derivatives, may be used. It should be understood that
these terms are not intended as synonyms for each other. Thus, the
scope of the expression a device A coupled to a device B should not
be limited to devices or systems wherein an output of device A is
directly connected to an input of device B. It means that there
exists a path between an output of A and an input of B which may be
a path including other devices or means. "Coupled" may mean that
two or more elements are either in direct physical, electrical or
optical contact, or that two or more elements are not in direct
contact with each other but yet still co-operate or interact with
each other.
[0185] Thus, while there has been described what are believed to be
the preferred embodiments of the disclosure, those skilled in the
art will recognize that other and further modifications may be made
thereto without departing from the spirit of the disclosure, and it
is intended to claim all such changes and modifications as fall
within the scope of the disclosure. For example, any formulas given
above are merely representative of procedures that may be used.
Functionality may be added or deleted from the block diagrams and
operations may be interchanged among functional blocks. Steps may
be added or deleted to methods described within the scope of the
present disclosure.
* * * * *